Exoatmospheric devices are devices which are traveling or located in the area of outer space near the Earth's atmosphere. These devices can include satellites, intercontinental ballistic missiles, space vehicles, etc. When in exoatmospheric flight, these devices do not experience any significant external acceleration forces. To navigate, these devices typically use three gyroscopes and three linear accelerometers. The three gyroscopes measure angular motion about 3 orthogonal axes and the three linear accelerometers measure translational movement along those 3 orthogonal axes.
Based on the data received from the gyroscopes and linear accelerometers, a navigation computer is able to determine position, velocity, and attitude which are used to navigate the exoatmospheric devices. Various methods are known for using this data in navigation. In addition, a navigation computer may use a Kalman filter to couple the data from the gyroscopes and linear accelerometers, as well as other sensors such as Global Positioning System (GPS) sensors.
However, sensor error in the data received from sensors, such as gyroscopes and/or linear accelerometers, can negatively affect the navigation calculations. For example, one source of error in measurements and navigation calculations is due to gyroscope scale factor error. Scale factor error is a constant difference between the expected and actual output of a sensor. This scale factor error also has a tendency to vary with temperature changes on the sensor. For example, a gyroscope scale factor of 5 parts per million (ppm) indicates that for every 1 million degrees of rotation detected by the gyroscope, the gyroscope will be inaccurate by up to 5 degrees. A gyroscope detecting rotation of an exoatmospheric device, which is rotating about a spin axis at an exemplary rate of 1000 degrees per second for 1000 seconds, with a 5 ppm scale factor error, would then exhibit an error of 5 degrees. This error adversely affects attitude and position measurements. Inaccurate measurements of attitude and position, in turn, adversely affect navigation of the exoatmoshperic device.
Exoatmospheric conditions make it difficult to detect and correct for some errors, such as gyroscope scale factor error, due to the lack of external acceleration forces. One method of dealing with gyroscope scale factor error is to use high accuracy gyroscopes on a spin axis. For example, one typical high accuracy gyroscope has a gyroscope scale factor of 0.3 ppm. By minimizing gyroscope scale factor, these high accuracy gyroscopes improve the accuracy of navigation calculations. However, these high accuracy gyroscopes can be very expensive and cost prohibitive.